Step-by-Step APT Guide for NIRCam WFSS Deep Galaxy Observations

Instructions are providing for filling out the JWST APT observing template for the NIRCam WFSS Deep Galaxy Observations example science program, consisting of NIRCam WFSS observations. The steps below roughly follow those given in NIRCam Wide Field Slitless Spectroscopy APT Template.

Dated material

This example was created pre-launch, and the APT has been updated since its creation. You may see differences in the appearance of the APT GUI and/or the warnings and errors in APT from what is shown herein.

Please refer to JWST Example Science Programs for more information.

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APT file

See also: NIRCam Wide Field Slitless Spectroscopy APT TemplateNIRCam WFSS Recommended StrategiesJWST APT HelpJWST APT Video Tutorials

A filled out APT file can be accessed via the APT menu options File → JWST Example Science Proposals → NIRCam → 37 NIRCam WFSS Deep Galaxy Observations. The APT file was created with version 27.3. There may be inconsistencies or additional warnings or errors with other versions of APT.

Fill out Proposal Information 

See also: JWST Astronomers Proposal Tool overview

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

After opening APT, we selected New JWST Proposal under the New Document pull-down menu. On the Proposal Information page, we entered Title, Abstract, and Category of proposal and kept Cycle number at its default value.

Enter proposed target

See also: APT Targets

This program is targeting SDSS J0100+2802, a z > 6 quasar at RA = 01 00 13.02, Dec = 28 02 25. In Targets, create a new Fixed Target with those coordinates and provide a name J0100+2802 (abbreviated as spaces are not allowed in APT target names).


See also: APT Observations

In Observations, create a new Observation Folder. Observation 1 will be created automatically. Select NIRCam Wide Field Slitless Spectroscopy as the Template and select the Target defined above.

  • Instrument: NIRCAM
  • Template: NIRCam Wide Field Slitless Spectroscopy
  • TargetJ0100+2802

Complete APT observation template for NIRCam WFSS observations

See also: NIRCam Wide Field Slitless Spectroscopy APT Template, NIRCam WFSS Recommended Strategies

Proceed to the NIRCam Wide Field Slitless Spectroscopy tab (leftmost tab on the lower half of the APT screen). Since a wide observing area is desired (saturation is not a concern for these faint targets), choose Module = ALL and Subarray = FULL to cover a larger field of view. For the Grism (long wavelength), select GRISMR as discussed in the introduction.

  • Module: ALL
  • Subarray: FULL
  • Grism (Long Wavelength): GRISMR

Dither patterns

See alsoNIRCam Primary DithersNIRCam Subpixel Dithers

As discussed in the intro article, this program uses INTRAMODULEX primary dithers and 4 standard subpixel dithers, for a total of 12 dithered exposures.

Dither Parameters:

  • Primary Dither Type: INTRAMODULEX
  • Primary Dithers: 3
  • Subpixel Positions: 4-Point

Science (grism) exposures

Enter the WFSS exposure parameters determined in our ETC Calculations (Calculation #1). Again note the full WFSS F356W exposure time is divided in 2: the first half with simultaneous F115W imaging and the second with F200W imaging.

Direct ImageLong FilterShort FilterReadout PatternGroupsIntegrationsETC Wkbk.Calc ID


The first column indicates that a single direct image will be obtained after the final WFSS exposure, as required. This program forgoes the option of obtaining an additional direct image after the first set of exposures (F356W GRISMR and F115W imaging).

Direct image exposures

Next, enter the direct image exposure parameters determined in our ETC Calculation #2.

Long FilterShort FilterReadout PatternGroupsIntegrationsETC Wkbk.Calc ID

Exposure sequence display

Given the inputs above, the display below will automatically populate showing the sequence of exposures. Note that after the direct image, 2 out-of-field images are obtained with the same exposure specifications.

SequenceExposure TypeLong WavelengthShort WavelengthReadout PatternGroupsDithersTotal Exposure Time
1Grism & ImagingGRISMR + F356WF115W imagingSHALLOW46123736 s
2Grism & ImagingGRISMR + F356WF200W imagingSHALLOW46123736 s
2Direct ImageF356W imagingF200W imagingSHALLOW4101526 s
2Out of FieldF356W imagingF200W imagingSHALLOW41021052 s

Define mosaic strategy

The above observation sequence will be repeated 4 times in a 2 × 2 mosaic, as described in the introduction. The mosaic tiles will be spread to cover more area and capture more high redshift galaxies surrounding the primary target quasar. But the mosaic tiles will also overlap such that every observation will include the quasar. The desired mosaic coverage is implemented as follows in APT and shown below in Figure 1.

Under the Mosaic Properties tab:

  • Rows: 2
  • Columns: 2
  • Row Overlap %: 45
  • Column Overlap %: 72
  • Row shift: 0
  • Column shift: -1.2° (rotation)

The overlap and shift values can be determined by trial and error after inspecting the results in APT Aladin. The final entry (column shift) makes the final mosaic a bit more symmetric (rectangular) as there are slight asymmetries in the NIRCam Field of View.

Figure 1. Mosaic to be observed (Long Wavelength channel)

APT screenshot of the final 2 × 2 mosaic, with each tile consisting of 12 dither positions, 1 direct image, and 2 out-of-field images. Darker regions correspond to greater exposure time in the long wavelength channel. The short wavelength observations are not shown, for clarity.

Define special requirements

See also: APT Special Requirements, JWST Position Angles, Ranges, and Offsets

To recenter the target within the mosaicked observations, a small offset of (4", -3.5") in (X, Y) is added via the Special Requirements tab (again determined by trial and error inspecting the output in APT Aladin).

Finally, the Aperture PA Range was restricted to include other targets of interest. Two options are provided: 60–65 degrees and 230–240 degrees.

Run Visit Planner

See also: APT Visit Planner

To determine the visibility window of our proposed observation and ensure that guide stars can be found, we run the Visit Planner Tool, by clicking the Visit Planner icon in the top toolbar, selecting the observation, and clicking Update Display. The Visit Planner confirms observable windows totaling over 2 months per year.

Note after updating the display, there is no need to run Smart Accounting (the option is grayed out).

View Observations in Aladin

See also: APT Aladin Viewer

In the top toolbar, click View in Aladin to see the observations oriented on the sky in a new window. Then in that window, click DSS in the top toolbar to see the observations overlaid on images from the Digital Sky Survey.

Note the orientation (rotation) is 62.5 degrees, the middle of the first range of position angles input as an option.

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